drm/i915/gvt: always use readq and writeq

The readq and writeq are already offered by drm_os_linux.h. So we can
use them directly whithout dectecting their presence. This patch removed
the duplicated code.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>